A series of passive gas diffusers and their accompanying piping system are frequently used to introduce air or other gases into a body of liquid in which they are submerged for oxygenation or other treatment of the liquid. Quite often a fairly large number of such gas diffusers is employed, with intervening pipes connecting the individual diffusers, and in such cases the entire system may extend for quite a distance. This is very often true when a natural body of water such as a small lake is being treated.
One of the problems with such diffusers and their accompanying piping systems is that when the diffusers are not in use the gas pressure falls within the gas plenums of the diffusers, and the liquid in which the diffusers are submerged flows into the supply pipes and diffusers.
In order to have gas introduced into the diffusers at the desired rate of flow, the supply pipes are commonly made with a rather large internal diameter to reduce pressure drop in the supply pipe. However, with conventional diffuser and piping systems optimum use is not made of the large diameter of such pipes. When a large part of the supply pipe remains filled with liquid following the shut-down during which liquid from outside the diffusers backs up in the system as just described, the effectiveness of the supply pipe is greatly reduced for the following reasons:
1. Gas flow in a pipe depends upon the cross-sectional area inside the pipe and the velocity of the gas in the pipe. Thus, for a given gas velocity the gas flow rate is reduced when the flow path area normal to the flow is reduced because of accumulated liquid in the pipes.
2. The pressure drop due to friction in the supply pipes for a given gas velocity depends upon pipe length and the pipe diameter. The pipe length is fixed upon installation. If the diameter available to gas flow decreases because liquid accumulates in the pipe, the pressure drop due to friction will increase.
3. The gas flow through a porous media gas diffuser is very sensitive to gas pressure in the plenum beneath the porous media, and a small change in the plenum gas pressure can effect a large change in the gas flow through the diffuser media. Thus an increase in the pressure drop in the gas supply pipe (such as referred to in paragraph 2 just above) can have a significant adverse effect on the gas flow rate through the diffuser.
These disadvantages are all present with conventional apparatus that is used to clear out unwanted accumulated liquid from the supply pipe for gas diffusers submerged in a liquid as described. In the conventional apparatus referred to, gas to be discharged from the diffusers flows into those diffusers from the top spaces in the supply pipe, and unwanted liquid is discharged either (1) from the bottom of the pipe or (2) out through a blowout assembly that extends upward through the top of the supply pipe to communicate with the atmosphere above the body of liquid being treated. The shortcomings of these two types of conventional apparatus will be seen from a discussion of the manner in which they operate.
The first device for the bottom draining of supply pipes just mentioned includes a spring-loaded ball float or other ball float or valve mechanism positioned at the bottom of the supply pipe, where it operates in conjunction with an opening in the bottom of the pipe through which liquid is forced out when the piping system is pressurized as the diffusers are started up. The ball float or other valve mechanism operates at an elevated pressure to produce liquid flow out of the supply pipes. Ordinarily there is only one such bottom draining opening in a gas supply pipe that extends for as much as 20 feet in length.
The blowout assembly commonly used to expel unwanted accumulated liquid comprises a kind of standpipe that extends from near the bottom of the gas supply pipe upward through the top of the supply pipe to communicate with the atmosphere above the body of liquid being treated. In usual practice, there may be only one of these blowout assemblies for a quite lengthy extent of gas supply pipe.
When gas is introduced at start-up into the supply pipe of either type of conventional system just described, after a certain pressure is reached in the space above the liquid in the supply pipes, gas flowing out the outlet at the top of the supply pipe into the gas plenum associated with the diffuser will pass from there out through the porous bubble-forming member. Consequently an extremely high (and thus expensive) gas pressure is required to produce a large enough gas flow to bring about the desired gas flow out of the diffusers and at the same time to maintain enough back pressure that unwanted accumulated liquid will be forced out the bottom of the supply pipe or out through the blowout assembly standpipe, as the case may be.
The high pressure required in each case just discussed will exceed the normal operating pressure for the gas diffuser by an additional pressure necessary to expel unwanted liquid against the hydrostatic pressure measured immediately outside and directly below the supply pipe in the case of bottom draining, or against the hydrostatic pressure measured at a level directly above the bottom wall of the gas supply pipe in the case of the described blowout assembly. This in turn will increase the gas flow rate out of the gas diffuser associated with the supply pipe to a value above the rate at which the diffuser is designed to operate, and this will result in an expensive waste of treating gas. (As used in this specification and claims, the term "normal operating pressure" of the gas diffuser is the pressure at which gas will pass out of the porous top wall of the diffuser in the volume required to maintain but not exceed the particular gas flow rate at which the diffuser is designed to operate.)
An additional disadvantage with conventional systems for removal of unwanted accumulated liquid results from the fact that with such systems, unless all the diffusers and their associated supply pipes are maintained within extremely close tolerances at exactly the same level within the body of liquid, the common level of accumulated liquid within the pipes may fill a considerably larger fraction of the pipes at some points in the series of diffusers than at others. Some of the individual diffusers will then operate at substantially lower efficiency than others. This problem is most acute in those instances mentioned above in which a large number of diffusers is employed and the resulting series of diffusers extends for quite a long distance.
The gas diffuser and piping system of the present invention avoids all these disadvantages.